This analysis examined associations between total reduced sulfur (TRS) and hydrogen sulfide (H(2)S) levels, and hospital visits for respiratory disease among residents of Dakota City and South Sioux City, Nebraska, from January 1998 to May 2000. For reference, the association between TRS, H(2)S, and digestive diseases was also examined. Time-series analyses of daily hospital visits in the selected outcome categories and measures of TRS and H(2)S were performed using generalized additive models with a Poisson link. TRS and H(2)S levels were categorized as high if at least one of the daily 30-min rolling averages was > or =30 ppb and as low if every rolling average was <30 ppb. Loess smoothers allowed for flexible modeling of the time effect and the effect of temperature and relative humidity. The measure of association used was the mean percent change in the average number of hospital visits recorded following a day with a high exposure versus a day with a low exposure. For children less than 18 years of age, a positive association was found between asthma hospital visits and 1-day lagged TRS levels. For adults, a positive association was found between asthma hospital visits and H(2)S levels on the previous day. A positive association also was found between hospital visits for all respiratory diseases, and H(2)S and TRS levels on the previous day for children but not for adults. No association was found between contaminant levels and hospital visits for all digestive diseases. These findings suggest that TRS or H(2)S levels may be associated with exacerbations of asthma or other respiratory diseases among the residents of Dakota City and South Sioux City.